Tool guide for a surgical tool

ABSTRACT

A patient is secured to a subject support ( 10 ). A stereotaxic wand ( 40 ) is inserted into a tool guide ( 60 ). The wand has a tip portion ( 44 ), a portion extending along a pointing axis ( 46 ) of the wand, an offset portion ( 42 ) which is offset from the pointing axis of the wand, and at least two wand emitters ( 48, 50 ), mounted in alignment with the pointing axis of the wand. The two emitters selectively emit wand signals which are received by three receivers ( 14 ) mounted to a frame assembly ( 12 ). The tool guide includes a bore ( 66 ) extending along a guide axis. The bore is configured for selectively receiving a tool and the tip portion of the wand. An entry point and a trajectory are identified by the surgeon with the wand in the guide. More specifically, a trajectory and location of the wand are superimposed on a diagnostic image on a monitor ( 30 ). If the surgeon is satisfied with the entry point and trajectory shown on the monitor, a surgical tool is inserted into the bore while the tool guide is held along the designated trajectory and at the designated entry point.

This application is a divisional of U.S. application Ser. No. 08/650,312filed May 20, 1996, now U.S. Pat. No. 5,776,064 which, in turn, is acontinuation of U.S. application Ser. No. 08/224,955 filed Apr. 8, 1994,now U.S. Pat. No. 5,517,990 which, in turn, is a continuation-in-part ofU.S. application Ser. No. 07/983,390 filed Nov. 30, 1992, now U.S. Pat.No. 5,309,913.

BACKGROUND OF THE INVENTION

The present invention relates to the medical diagnostic and surgicalarts. It finds particular application in conjunction with neurosurgeryand will be described with particular reference thereto. However, it isto be appreciated, that the invention will also find application inconjunction with neurobiopsy, CT-table needle body biopsy, breastbiopsy, endoscopic procedures, orthopedic surgery, other invasivemedical procedures, industrial quality control procedures, and the like.

A three-dimensional diagnostic image data of the brain, spinal cord, andother body portions is produced by CT scanners, magnetic resonanceimagers, and other medical diagnostic equipment. These imagingmodalities typically provide structural detail with a resolution of amillimeter or better.

Various frameless stereotactic procedures have been developed which takeadvantage of three-dimensional image data of the patient. Theseprocedures include guided-needle biopsies, shunt placements,craniotomies for lesion or tumor resection, and the like. Another areaof frameless stereotaxy procedure which requires extreme accuracy isspinal surgery, including screw fixation, fracture decompression, andspinal tumor removal.

In spinal screw fixation procedures, for example, surgeons or othermedical personnel drill and tap a hole in spinal vertebra into which thescrew is to be placed. The surgeon relies heavily on his own skill inplacing and orienting the bit of the surgical drill prior to forming thehole in the vertebra. Success depends largely upon the surgeon'sestimation of anatomical location and orientation in the operativefield. This approach has led to suboptimal placement of screws that mayinjure nerves, blood vessels, or the spinal cord.

The present invention provides a new and improved technique whichovercomes the above-referenced problems and others.

SUMMARY OF THE INVENTION

In accordance with the present invention, an apparatus guides a surgicaltool in relation to a patient. The apparatus includes a guide memberwhich defines a guide axis and a means for generating signals indicativeof an orientation of the guide axis. The guide member is configured tosupport a tool along the defined guide axis.

According to another aspect of the present application, the tool guideincludes a grooved portion defined along the guide axis. The groove isconfigured to receive the tool.

One advantage of the present application is that it facilitates moreaccurate surgical procedures.

Another advantage of the present invention is that it promotes patientsafety.

Still further advantages of the present invention will become apparentto those of ordinary skill in the art upon reading and understanding thefollowing detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating a preferred embodiment and are notto be construed as limiting the invention.

FIG. 1A is a perspective view of an operating room in which the presentinvention is deployed;

FIG. 1B is a block diagram of the image data manipulation of the systemof FIG. 1A;

FIGS. 2A, 2B, 2C, 2D, 2E, 2F, and 2G illustrate alternate embodiments ofthe wand and guide;

FIG. 3 is a detailed illustration of the locator assembly of FIG. 1;

FIG. 4 is a diagrammatic illustration of one embodiment of calibrationprocedure in accordance with the present invention;

FIGS. 5A and 5B are diagrammatic illustrations of the wand and locatorrelationship;

FIGS. 5C is a flow diagram of the wand location procedure;

FIGS. 6A, 6B, 6C, and 6D are illustrative of a preferred coordinatetransform between the coordinate system of the data and the patient.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1A, a subject, such as a human patient, isreceived on an operating table or other subject support 10 andappropriately positioned within the operating room. A frame 12 ismounted in a fixed relationship to the patient such that it is preciselypositioned within the subject or subject support coordinate system. Inthe illustrated embodiment, the frame 12 is mounted to the patientsupport 10. Mounting the frame 12 to the patient support permits thepatient support to be turned, raised, lowered, wheeled to anotherlocation, or the like, without altering the patient coordinate system.Alternately, the support may be mounted to a pole or other stationarysupport, the ceiling of the room, or the like. The frame 12 supports aplurality of receivers 14 such as microphones, radio frequencyreceivers, light sensitive diodes, other light sensitive receivers, andthe like mounted at fixed, known locations thereon. A securing meanssuch as a head clamp 16, securely positions a portion of the subjectunder consideration. The frame is mounted at a fixed or selectable anglefrom vertical such that the frame is positionable more toward thepatient, yet still focusing on the region of interest of the patient.

With continuing reference to FIG. 1A and further reference to FIG. 1B,an operator console 18 houses a computer system 20. Alternately, thecomputer system can be remotely located and connected with the controlconsole 18 by cabling. The computer system includes a three-dimensionaldata memory 22. The stored three-dimensional image data preferablycontains a video pixel value for each voxel or point in athree-dimensional rectangular grid of points, preferably a 256×256×256grid. When each image value represents one millimeter cube, the imagedata represents about a 25.6 centimeter cube through the patient withone millimeter resolution. Because the data is in a three-dimensionalrectangular grid, selectable orthogonal and other oblique planes of thedata can readily be withdrawn from the three-dimensional memory usingconventional technology. A plane selecting means 24 selects varioustwo-dimensional planes of pixel values from the three-dimensional memoryfor display.

The plane selecting means selects at least three planes: axial,sagittal, coronal, and oblique planes through a selectable point of thepatient. The pixel values which lie on the selected axial, sagittal,coronal, and oblique planes are copied into corresponding image memories26 a, 26 b, 26 c, and 26 d. A video processor means 28 converts thetwo-dimensional digital image representations from one or more of imagememories 26 into appropriate signals for display on video monitors 30 orother appropriate display means.

With continuing reference to FIG. 1A and further reference to FIG. 2A, awand 40, formed of suitable resilient material such as metal, has anoffset 42 near a tip portion or proximal end 44. The offset 42 isconnected to a portion extending along a pointing axis 46 of the wand.In this preferred embodiment, a pair of emitters 48 and 50 are mountedon the offset and disposed along the pointing axis 46 of the wand. Theemitters emit a signal that is received by the receivers 14, such as aspark emitter, radio frequency transmitter, infrared LED, or the like.The first emitter 48 is a fixed known distance l₁ from the tip 44 andthe second emitter 50 is a fixed known distance l₂ from the firstemitter 48. The wand is readily sterilized by conventional techniques.Emitters 48 and 50 emit positioning signals used by a locator system 52to locate a coordinate and trajectory of the wand. The plane selectingmeans 24 selects patient image planes based on the coordinate andtrajectory located. It is to be appreciated that more than two emittersmay be mounted on the offset to provide additional positioning signalsto be used by a locator system to locate the coordinate and trajectoryof the wand.

The wand 40 is used in conjunction with a guide 60 to designate acoordinate and trajectory at which a surgical tool will be applied tothe patient. The guide can be any guide or appliance which positions thewand 40 to establish a surgical trajectory. In the preferred embodiment,the guide 60 is a portable tool which has a handle 62 and a tube member64 which defines an internal bore 66 to receive and accurately positionthe wand. The bore 66 of the tool guide has a diameter which allows forthe non-simultaneous insertion of either the wand 40 or a surgical toolsuch as a drill, biopsy needle, and the like. Rather than beinghand-held, the guide 60 can be mounted to other structures in theoperating room, e.g. framed stereotaxic equipment. In intraoperativeuse, the wand 40 is inserted in the tool guide bore until the tip 46aligns with a tool guide end 68. A wand stop 70 is positioned on thewand and abuts a distal end surface 72 of the tool guide when the wandtip aligns with the proximal end 68. With the wand tip aligned with thetool guide end, the surgeon may commence probing the patient to seek anoptimal coordinate and trajectory in which to insert the appropriatesurgical tool. To this end, the surgeon maneuvers the wand and toolguide in combination to a proposed trajectory and actuates the emitters.Signals from the emitters and used in calculating the trajectory and theend point 68 of the wand. The trajectory and end point are displayed onthe monitor 30 superimposed on the three-dimensional image or selectedimage planes. The details of the process for correlating the coordinatesystem of the patient and the wand with the coordinate system of theimage data is described below.

By viewing the display 30, the surgeon can identify the location of thewand tip with respect to anatomic structure, and the trajectory of thebore. If the trajectory is satisfactory, the wand is removed, thesurgical tool is inserted, and the procedure is performed. If thetrajectory is unsatisfactory, the wand is repositioned and its newtrajectory determined and evaluated. This. approach improves surgicalplanning when compared with prior approaches in which surgeons reliedsolely on their own estimation of the patient's anatomy. Following theidentification of the appropriate trajectory and coordinate, the wand 40is removed from the bore 66 of the guide 60 while the guide is held inposition. Holding the guide 60 steady preserves the appropriatetrajectory and position coordinates in the axial and sagittal planesdetermined by the wand. Thereafter, the appropriate surgical tool orappliance is inserted within the guide 60. With this approach, thesurgical tool is properly positioned in the appropriate trajectoryrequired to perform the surgical procedure.

The wand and tool guide are particularly useful in accuratelyidentifying the optimal entry point, trajectory, and depth of insertionof screws to be placed into the patient's spinal column, as will be morefully described below.

With reference to FIGS. 2B, 2C, and 2D alternative embodiments of thepresent invention are shown in which the guide is integrated into thewand. In general, each of the alternative embodiments contain a wandoffset portion on which are mounted two or more emitters for emittingpositioning signals. As in the preferred embodiment, emitters 48 and 50are disposed along a pointing axis 46 of the wand. However, in thealternative embodiments, the central axis or pointing direction 46aligns with a longitudinal axis of the guide means formed integrallywith the wand. In each of the alternative embodiments, the guide meansis connected to the wand offset portion via an extension.

With reference to FIG. 2B, a tubular portion 74 is integrated with thewand. The tubular portion defines a bore 76 extending along itslongitudinal axis. In intraoperative use, the surgeon probes the patientwith the proximal seeking to locate the proper coordinate and trajectoryfor the surgical tool. Once the coordinate and trajectory are located,the surgeon holds the offset portion while the surgical tool is insertedwithin the tube. Thereafter, the surgical tool is operated to performthe surgical procedure.

With reference to FIG. 2C, a second alterative embodiment is shownsimilar to the previously described alternative embodiment. However, inaddition to the structure previously described, a laser 78 is mounted tothe offset portion. Light emitting from the laser travels along thelongitudinal pointing axis 46 of the bore 66 of the tubular member. Inintraoperative use, the surgeon maneuvers the integrated wand and laserwhile viewing images displayed on the monitor 30. The images selectedfor display are based upon the coordinate and trajectory of the borecenter point at the proximal end of the integrated wand. Once a propercoordinate and trajectory are identified, the integrated wand is held inplace while the surgeon activates the laser. Light emitting from thelaser intersects the bore center point at the proximal end.

With reference to FIG. 2D, a third embodiment is shown in which agrooved member 80 is incorporated into the wand. The grooved member isconnected to the offset portion via an extension 82. The grooved membercontains a groove 84 having a longitudinal axis which is in line withpointing axis 46 of the wand. This alternative embodiment findsparticular usefulness in conjunction with needle biopsies. Inintraoperative use, a biopsy needle 86 is positioned within the grooveso that a tip 88 of the biopsy needle aligns with the groove centerpoint at the proximal end of the integrated wand. The biopsy needle isheld in place by a restraining means such as Velcro® straps 88 attachedto the sides of the grooved member.

In the embodiment of FIG. 2E, the probe 40 has emitters 50, 50′, 50″mounted off the axis 46. Because the relationship between the emitterlocation and the axis 46 is fixed, once the emitters are located, theaxis 46 is determined.

In the embodiment of FIG. 2F, there are more than two emitters 50 ₁, 50₂, 50 ₃, . . . Although any two emitters would determine the axis 46,greater accuracy is obtained by redundantly determining the axis 46 andaveraging the results. Preferably, a least squares fit is used tocompensate for any deviation in the axis 46 determined by the variousemitter pairs.

In the embodiment of FIG. 2G, the probe 40 has interchangeable tips. Thewand 40 includes a first connector portion 90 which selectively connectswith a second connector portion 92 of the tips. Various connectorsystems are contemplated such as a threaded bore and threaded shaft, asnap lock connector means, bayonet connector, spring lock, or otherconnector systems. A keyway 94 or other means for fixing the alignmentof the tips and the wand is particularly advantageous when the connectoris off the axis 46.

Various tips are contemplated. A short tip 96 is provided for accurateregistration. A longer tip 98 facilitates reaching deeper into interiorregions of the subject. Tubular drill guides 100 can be provided invarious diameters to accommodate different size drills. An adapter 102enables endoscopes and other tools to be attached to the wand. Tools andequipment, such as an array of ultrasonic transducers 104, can beconnected to the adaptor 102 or configured for direct connection to thewand. A wide variety of other tips for various applications are alsocontemplated.

The preferred embodiment uses the stereotaxic wand 40 to align thecoordinate system of the operating room including the patient, the toolguide, and wand with the coordinate system of a previously preparedthree-dimensional image stored in memory. Prior to identifying theproper coordinate and trajectory of the tool guide, the patient space isaligned with or referenced to the stored three-dimensional image datapreferably using the following technique.

With reference to FIG. 3, when the receivers 14 are microphones, aplurality of reference emitters 106 are mounted on the frame 12. Thereference receivers are each spaced along side edges of the frame byknown distances from adjacent receivers or microphones 14, e.g. bydistances S₁ and S₂. Preferably S₁=S₂=S. Each reference receiver is alsospaced by a distance D across the frame from an oppositely disposedemitter.

The distance from the wand emitters to the frame, hence the position ofthe wand relative to the patient, is determined by the travel time ofthe sound. The velocity of the sound pulse through air is dependent uponboth the temperature, the humidity, and the chemical composition of theair. These factors can and do vary significantly during an operation andfrom procedure to procedure. As shown in FIG. 4, a calculation isperformed to determine the speed of sound in the operating room. Acalibrating means 110 selectively pulses the reference emitters 106,receives the signals at microphone receivers 14, and processes theelapsed time information in accordance with the procedure of FIG. 4.More specifically, the calibration means 110 includes a step or means112 for causing a first of the reference emitters 100 to emit a signalpulse. A step or means 114 acquires the range values D′, i.e. the timerequired for the ultrasonic pulses to traverse the distance D. A step ormeans 116 causes this procedure to be repeated a preselected number oftimes, such as once for each of the four emitters illustrated in FIG. 3.

Once the travel time between each emitter and receiver pair has beenobtained a preselected number of times, a step or means 120 corrects thetimes for fixed machine delays. That is, there is a fixed, small delaybetween the time when the command is given to fire the referenceemitters 106 and the time that they actually produce a detectableultrasonic signal. Analogously, there is a small delay between the timethat the ultrasonic pulses reach the receiver or microphone 14 and thetime that it becomes a measurable electrical signal received by thecomputer processor. These delays are subtracted from the times measuredby step or means 114. An averaging means 122 averages the actual timesafter correction for the machine delays for transmission of theultrasonic pulse between the transmitter and receiver. The time over therange values D′ provide the most accurate results. A step or means 124computes a calibration factor F indicative of the current speed of theultrasound signal adjacent the patient in the operating room. In thepreferred embodiment, the calibration factor F is a ratio of the soniclymeasured distance D′ versus a precise mechanical measurement of thedistance D.

With reference to FIGS. 5A, 5B, and 5C, a wand coordinate and trajectorydetermining means 130 determines the position of the two emitters 48 and50, respectively. More specifically, a step or means 132 causes theemitter 48 to emit an ultrasonic signal. The receivers 110 on the frame12 receive the ultrasonic signal at corresponding times L₁-L₄. A step ormeans 134 acquires and retains these times. A step or means 136 causesthe second emitter 50 to transmit. A step or means 138 acquires the fourtimes L₁-L₄ which are required for the ultrasonic signals to pass fromthe second emitter to the microphones 14. The speed of ultrasonictransmission and accuracy of transmission times are such that thesedistances can be measured to within a millimeter or better. A step ormeans 140 causes the emitters to emit and corresponding data valuesL₁-L₄ to be acquired each of a plurality of times to improve digitationaccuracy, e.g. two times.

When sonic emitters are used, a step or means 142 causes the calibrationmeans 110 to perform the steps described in conjunction with FIG. 4 inorder to provide a current indication of the velocity of sound adjacentto the patient. Of course, the calibration procedure of FIG. 4 may beperformed immediately before steps 132-138 or intermittently during thecollection of several data values for averaging. A step or means 144corrects the values L₁-L₄ for the fixed machine delay discussed above inconjunction with step or means 120. A step or means 146 corrects each ofthe times L₁-L₄ that were required for the ultrasonic signals to travelfrom the first and second emitters 48, 50 to the receivers 14 inaccordance with the correction factor F determined by step or means 124.An averaging means 148 averages the delay and calibration correctedtimes L₁-L₄, hence distances between each of the wand emitters 48, 50and each of the receivers 14. From these distances, provided at leastthree receivers 14 are provided, a step or means 150 calculates theCartesian coordinates (x₁,y₁,z₁) and (x₂,y₂,z₂) in the patient space forthe two emitters 48 and 50. The first emitter coordinates x₁,y₁,z₁ arecalculated from three of the four range values L₁-L₄. With L₄disregarded, the coordinates are calculated as follows:

x ₁=[(L ₁ ² −L ₂ ²)+S ²]/2S  (1a),

y ₁=[(L ₁ ² −L ₃ ²)+S ²]/2S  (1b),

 z ₁ =[L ₁ ² −x ₁ ² −y ₁ ²]^(½)  (1c)

where S=S₁=S₂ as defined in FIG. 3. Preferably, the three selected rangevalues are the three shortest of L₁-L₄. Similar computations arecalculated for x₂, y₂, and z₂ coordinates of the second emitter. A stepor means 152 checks the validity of the measurement. More specifically,the known separation between the wand emitters is compared with theseparation between the measured coordinates x₁,y₁,z₁ and x₂,y₂,z₂ of thewand emitters, i.e.:

|Sep_(known)−[(x ₁ −x ₂)²+(y ₁ −y ₂)²+(z ₁ −z ₂)²]^(½)|≦error.  (2).

If the measured and known separation is greater than the acceptableerror, e.g. 0.75 mm when measuring with a resolution of 1 mm, anerroneous measurement signal is given. The measurement is discarded andthe surgeon or other user is flagged to perform the measurement process130 again. A step or means 154 from the coordinates of the two emitters48, 50, and from the geometry of the wand discussed in FIG. 2,calculates the Cartesian coordinates (x₀,y₀,z₀) for the wand tip 46.

The tip coordinates x₀, y₀, z₀ are defined by:

r=l ₁ /l ₂  (3a),

x ₀=(1+r)x ₁ −rx ₂  (3b),

y ₀=(1+r)y ₁ −ry ₂  (3c),

z ₀=(1+r)z ₁ −rz ₂  (3d).

Before the wand and tool guide can be used to locate a proper coordinateand trajectory for a surgical tool such as a drill, the patient space isaligned with the image space stored in memory. Aligning the spacesbegins with referencing known positions in the patient space with thewand tip. For example, the tip 46 of the wand may be referenced to threeindependent positions of the vertebra, i.e. the tips of the spinous andtraverse processes. These positions on the vertebra are compared withthe relative position of pixels 160 in the image space. Thereafter, withreference to FIG. 6, a transform means 162, as shown in FIG. 6,transforms the coordinates of the patient space into the coordinatesystem of the image space. Fiducials can also be used by the transformmeans 162 to transform or match the coordinates of other patient spaceportions into the coordinate system of the image space. To this end,three or more fiducials or markers are affixed at three or more spacedpoints on the patient's body. The fiducials are visible in the imagingmedium selected such that they show up as readily identifiable dots 164in the resultant image data. The fiducials are markers or small beads166 that are injected with radiation opaque and magnetic resonanceexcitable materials. A small dot or tattoo is made on the patient's skinand a fiducial is glued to each dot. This enables the position of thefiducials to be denoted even if the fiducials are removed in theinterval between the collection of the image data and the surgicalprocedure. To align the images of the fiducials with the fiducialpositions in patient space, the tip of the wand is placed on eachfiducial or tattooed marker point. The coordinates in patient space ofeach vertebra process tip or fiducial are determined with the proceduredescribed in conjunction with FIGS. 5A-5C.

The position of the three fiducials or process tips are compared withthe relative position of the pixels 160 in the image space. The patientspace coordinates of marks 166 or the process tips of the patient in thecoordinate system of the patient support are measured. A like coordinatesystem through the pixels 160 is defined and compared to the patientspace coordinate system. The translation and rotational relationshipbetween image space and patient space coordinate systems is determined.With reference to FIG. 6A, the position of the patient in operating roomspace (x′,y′,z′) and the relative position in image space (x′,y′,z′) aredetermined. That is, two coordinate systems are defined. The translationmeans first determines the offset x_(offset), y_(offset), z_(offset)between the barycenters 168, 170 of the triangles defined by thecoordinates of three fiducials or process tips in data and patientspace, respectively. This provides a translation or an offset in the x,y, and z-directions between the two coordinate systems. The values ofx_(offset), y_(offset), and z_(offset) are added or subtracted to thecoordinates of the patient space and the coordinates of image space,respectively, to translate between the two.

With reference to FIG. 6B, translating the origins of the two coordinatesystems into alignment, however, is not the complete correction. Rather,the coordinate systems are normally also rotated relative to each otherabout all three axes whose origin is at the barycenter. As illustratedin FIGS. 6B, 6C, and 6D, the angle of rotation in the (y,z), (x,z), and(x,y) planes are determined. Having made these determinations, it is asimple matter to transform the patient support space coordinates intothe image space coordinates and, conversely, to rotate the image spacecoordinates into patient space coordinates. The wand coordinate means130 is connected through the transform means 162 with one of the planeselecting means 24 and the video processor 28 to cause a marker, e.g.cross hairs, to be displayed on the monitors 30 at the coordinates ofthe wand tip. This enables the surgeon to coordinate specific points onthe patient or in the incision with the images.

Having aligned the image and patient spaces the wand and tool guide canbe used to identify the entry coordinate and trajectory at which thesurgical tool will be applied to the patient. For example, the surgeonmay use the wand and tool guide in combination to identify thetrajectory and coordinate on the spinal column at which the surgeon willutilize a surgical drill in order to drill a hole for the placement of aspinal screw. Holding the wand and drill guide in one hand, the surgeonmoves the combination around the exposed vertebra while viewing imagesdisplayed on a monitor selected in accordance with the wand tip. Theimage provide a cross-sectional view of the vertebra and allow thesurgeon to plan with greater accuracy the angle and depth at which thedrill will be operated. Once the coordinate and trajectory of the drillapplication is identified, the surgeon may remove the wand while holdingthe tool guide in place. Since the tool guide comes with a handle, thesurgeon can hold the tool guide in place even when the spinal columnmoves in response to patient breathing. In other words, the surgeon caneasily hold the bore of the tool guide at the trajectory identified evenwhile the spinal column experiences movement. With the guide properlyoriented, the surgeon inserts into the bore the surgical tool and tipneeded for the spinal screw fixation. This technique is superior overprior methods in which surgeons relied solely on their own skill andknowledge of the patient's unique anatomy and will result in far fewersub-optimal results.

The present invention is also useful in preplanning a surgicaloperation. For example, surgeons may use images of the body portion atwhich the surgical tool will be inserted in order to determine prior tothe operation, the best trajectory and coordinate at which the toolshould be applied. Once the proper trajection and coordinate areidentified, the computer system can store these images in memory to belater used as a reference target to be compared with images produced inconnection with the wand and drill guide. In particular, the imagesproduced in accordance with the wand and drill guide could be comparedwith the stored images until the computer identifies an image match.Once the computer identifies a match, the computer can output a signalto alert the surgeon that the tool guide has been properly oriented.

The invention has been described with reference to the preferredembodiment. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

Having thus described the preferred embodiment, the invention is nowclaimed to be:
 1. An apparatus for guiding a surgical tool in relationto a patient, the apparatus comprising: a guide member defining a guideaxis, the guide member being configured to support a tool along thedefined guide axis; and a plurality of discrete sources, which arelocated on the guide member, of one of sonic, infrared, light, and radiofrequency signals indicative of an orientation of the guide axis.
 2. Theapparatus for guiding a surgical tool according to claim 1, wherein theguide member includes a bore extending along the guide axis, the borebeing adapted to receive the surgical tool.
 3. An apparatus for guidinga surgical tool in relation to a patient, the apparatus comprising; aguide member defining a guide axis, the guide member being configured tosupport a tool along the defined guide axis; and a means for generatingsignals indicative of an orientation of the guide axis, the signalgenerating means including: a wand which is configured for receipt inthe guide member; and at least two emitters affixed to the wand foremitting the orientation signals.
 4. An apparatus for guiding a surgicaltool in relation to a patient, the apparatus comprising: a guide memberdefining a guide axis, the guide member being configured to support atool alone the defined guide axis and including a groove extendingparallel to the guide axis to receive the surgical tool; and a means forradiating one of sonic, infrared, light, and radio frequency signalsindicative of an orientation of the guide axis.
 5. An apparatus forguiding a surgical tool in relation to a patient, the apparatuscomprising: a guide member defining a guide axis, the guide member beingconfigured to support a tool along the defined guide axis; and a meansfor generating signals indicative of an orientation of the guide axis,the signal generating means including at least two emitters mounted in afixed relationship to the guide member.
 6. An apparatus for guiding asurgical tool in relation to a patient, the apparatus comprising: aguide member defining a guide axis, the guide member being configured tosupport a tool along the defined guide axis; a means for generatingsignals indicative of an orientation of the guide axis; a means forreceiving the generated signals; a means connected to the means forreceiving for determining an orientation of the guide axis; a means forstoring a diagnostic image of the patient, the image having an imagecoordinate system; and a means for correlating the guide axis with theimage coordinate system.
 7. The apparatus for guiding a surgical toolaccording to claim 6 wherein the means for receiving includes one oflight sensitive receivers, microphones, and radio frequency receivers.8. An apparatus for guiding a surgical tool in relation to a patient,the apparatus comprising: a guide member defining a guide axis, theguide member being configured to support a tool along the defined guideaxis; and three signal emitters, which generate signals indicative of anorientation of the guide axis, on the guide member.
 9. A method forguiding a surgical tool in relation to a subject, the method comprising:generating signals indicative of an orientation of a guide axis definedby a guide member which is configured to support a tool along thedefined guide axis; storing image data indicative of a three dimensionalregion of a portion of the subject in an image memory; selecting planarslices of data from the image memory; converting the selected slices ofdata into human-readable displays; and transforming a position of a tipportion of a wand received in the guide member into a coordinate systemof the image data stored in the image memory, such that the displayedimages have a selected relationship to a position of the wand.
 10. Asystem for positioning a surgical tool comprising: a surgical tool; asurgical tool positioner having a bore for receiving and fixing alocation and an orientation of the surgical tool; and a plurality ofsignal sources each of which radiates one of sonic, infrared, light, andradio frequency signals and is positioned on at least one of thesurgical tool and the surgical tool positioner, the signal sourcesidentifying an orientation of the received surgical tool.
 11. The systemaccording to claim 10, wherein the bore has a circular cross sectionsuch that a positioning axis is defined centrally therealong.
 12. Thesystem according to claim 10, wherein the surgical tool is a drill. 13.A system for positioning a surgical tool, comprising: a surgical tool; asurgical tool positioner having a bore for receiving and fixing alocation and an orientation of the surgical tool; and a signalgenerator, including at least two emitters on at least one of thesurgical tool and the surgical tool positioner, for generating signals,which identify an orientation of the received surgical tool.
 14. Asystem for positioning a surgical tool comprising: a surgical tool; asurgical tool positioner having a bore for receiving and fixing alocation and an orientation of the surgical tool; and three signalemitters for generating signals which identify an orientation of thereceived surgical tool, the emitters being disposed on at least one ofthe surgical tool and the surgical tool positioner.
 15. The systemaccording to claim 14, wherein the surgical tool positioner includes astop surface that mates with a surface on the surgical tool to fix adistance that the tool is receivable into the bore.
 16. The systemaccording to claim 14, wherein the surgical tool is a biopsy needle. 17.A system for determining a trajectory defined by a tool guide, thesystem comprising: a tool guide; a subject support; a frame assemblywhich mounts a plurality of receivers in a fixed relationship to thesubject support; at least one of a pointer and a surgical tool; at leasttwo emitters mounted on the tool guide, the at least two emittersselectively emitting signals which are received by the plurality ofreceivers, the tool guide defining a channel which selectively receivesat least one of the pointer and the surgical tool such that at least oneof the pointer and the tool is aligned with a trajectory defined by thetool guide; and a position determiner for determining the trajectorydefined by the tool guide from the emitted signals traveling between theemitters and the plurality of receivers.
 18. The system according toclaim 17, further including: a three-dimensional image memory forstoring image data indicative of a three-dimensional region of a portionof a subject which is disposed on the subject support; a plane selectorfor selecting planar slices of data from the three-dimensional, imagememory; a display for converting the selected slices of data from theplane selector into human-readable displays; a transformer fortransforming the trajectory defined by the tool guide into a coordinatesystem of the image data stored in the three-dimensional image memory,the transformer being operatively connected with the plane selector suchthat the displayed images have a preselected relationship to the definedtrajectory.
 19. The system according to claim 18 wherein the pointer isreceived in the tool guide, the system further including: at least threemarkers adapted to be disposed on selected portions of a subject,locations of the three markers being identifiable in thethree-dimensional image data; and a transform calculator for calculatinga transform between positions of the markers when disposed on thesubject denoted by selectively placing the wand pointer on each of themarkers with the marker locations in the three-dimensional image data,whereby translational and rotational relationships between a subjectspace coordinate system and a three-dimensional image data spacecoordinate system are calculated.
 20. An apparatus for guiding anapplication of a tool to a subject from an entry point along atrajectory, the apparatus comprising: a guide member defining a guideaxis and having a distal end positionable on a selected subject entrypoint, said guide member being configured to support a tool along theguide axis; a means for holding, the distal end of the guide member atthe selected entry point and for holding the guide axis steady; asignaling means for sending one of sonic, infrared, light, and radiofrequency signals indicative of the guide axis from the guide member toa receiving means; a receiving means receiving the one of sonic,infrared, light, and radio frequency signals and determining anorientation of the guide axis; a means for storing a diagnostic image ofthe subject; a correlating means for correlating the determinedorientation of the guide axis with a coordinate system of the diagnosticdata; and a display means for displaying a selected portion of thediagnostic data with the guide axis superimposed thereon.
 21. Anapparatus for guiding an application of a tool to a subject from anentry point along a trajectory, the apparatus comprising: a guide memberdefining a guide axis and having a distal end positionable on a selectedsubject entry point, said guide member being configured to support atool along the guide axis; a means for holding, the distal end of theguide member at the selected entry point and for holding the guide axissteady; a signaling means for sending signals indicative of the guideaxis; a means for receiving the signals and determining an orientationof the guide axis; a means for storing a diagnostic image of thesubject; a correlating means for correlating the determined orientationof the guide axis with a coordinate system of the diagnostic data; and adisplay means for displaying a selected portion of the diagnostic datawith the guide axis superimposed thereon.